The present application claims priority to Japanese patent application, No. JPAP 2003-397683 filed on Nov. 27, 2003, the entire contents of which are incorporated by reference herein.
1. Field of the Invention
The present invention relates to a method and apparatus for power supply controlling. In particular, the present invention relates to a method and apparatus for power supply controlling capable of effectively controlling switching operations.
2. Description of the Related Art
In recent years, energy saving has strongly been demanded for environmental reasons. In a portable information handling apparatus using batteries, such as a cellular phone, mobile personal information terminal, digital camera, laptop personal computer, and so forth, a reduction of power consumption used in the above-described apparatus becomes more and more important, for example, to make the batteries last longer. As a result, a non-insulative step-down switching regulator is widely used as a power supply circuit. The non-insulative step-down switching regulator, which is hereinafter referred to as a “switching regulator”, employs an inductor and is highly efficient and downsizable.
However, while being highly efficient in a rated load, the switching regulator consumes a relatively large amount of power for driving itself, resulting in a significantly low efficiency when an apparatus is in a mode consuming lower power than in a normal operation mode. The mode consuming lower power is referred to as a “light load operation mode”, representing a stand-by mode and a sleep mode.
To increase efficiency of the above-described operation in the light load operation mode, some techniques have been proposed.
Referring to FIG. 1, a structure of a background circuit of a switching power supply circuit 101 employing one of these techniques is described. The technique uses a transistor having a short switching period in the light load mode to increase power supply efficiency in the light load operation mode. By using the above-described technique, the switching power supply circuit 101 may reduce power to be used in a transition period in which a switching element changes its status from power on to power off or from power off to power on.
Details of components provided in the switching power supply circuit 101 will be described.
In FIG. 1, the switching power supply circuit 101 includes a switching circuit 102, an outputting circuit 103 and a control circuit 104.
The switching circuit 102 includes a normal switching element 102a having a NPN-type (negative-positive-negative-type) transistor and a light load switching element 102b having a NPN-type transistor. The normal switching element 102a and the light load switching element 102b are connected in parallel between a collector serving as an input terminal E and an emitter serving as an output terminal B of the switching circuit 102.
The normal switching element 102a generates an applicable current in the normal operation mode. The light load switching element 102b generates a current having an amount smaller than that of the normal switching element 102a and has a switching time period shorter than that of the normal switching element 102a. The switching power supply circuit 101 drives the normal switching element 102a in the normal operation mode, and the light load switching element 102b in the light load operation mode, so that the above-described switching operations of the normal switching element 102a and the light load switching element 102b may convert a direct current voltage output by a direct current power supply 105 to a predetermined pulse to be output.
The outputting circuit 103 smoothes the predetermined pulse output by the switching circuit 102 and includes a diode 103a, coil 103b and a smoothing capacitor 103c. The diode 103a has an anode connected to the output terminal B on an output line of the switching circuit 102 and a cathode connected to a ground voltage. The coil 103b is connected on the output line of the switching circuit 102, between the cathode of the diode 103a and an input terminal C of a load current detection circuit 104f, which will be described below. The smoothing capacitor 103c has two electrodes on the output line of the switching circuit 102, one of which is connected to an output terminal D of the load current detection circuit 104f and the other is connected to a ground voltage.
The control circuit 104 feeds back a direct current voltage output from the outputting circuit 103, and controls a duty of the switching operations of the switching circuit 102 to stabilize the direct current voltage. The control circuit 104 also determines whether an operation mode of an apparatus is the normal operation mode or the light load operation mode to perform the switching operations of the switching circuit 102. That is, the control circuit 104 controls the switching operations between the normal switching element 102a and the light load switching element 102b of the switching circuit 102.
The control circuit 104 includes a differential amplifier 104a, reference voltage source 104b, PWM (pulse width modulation) comparator 104c, an oscillator 104d, a switching board 104e and the load current detection circuit 104f. 
The differential amplifier 104a includes a non-inverting input terminal and an inverting input terminal. The non-inverting input terminal of the differential amplifier 104a is connected to an output line of the outputting circuit 103 and the inverting input terminal of the differential amplifier 104a is connected to a positive terminal of the reference voltage source 104b. A direct current voltage is generated between the reference voltage source and a ground voltage.
The PWM comparator 104c includes a non-inverting input terminal and an inverting input terminal. The non-inverting input terminal of the PWM comparator 104c is connected to an output terminal of the differential amplifier 104a. The inverting input terminal of the PWM comparator 104c is connected to an output terminal of the oscillator 104d that generates a triangular wave according to a predetermined frequency.
The switching board 104e includes two input terminals and two output terminals. One of the input terminals of the switching board 104e is connected to the output terminal of the PWM comparator 104c and the other input terminal, which is input terminal A, is connected to an output terminal of the load current detection circuit 104f. One of the output terminals of the switching board 104e is connected to a base of the normal switching element 102a and the other is connected to a base of the light load switching element 102b. 
The load current detection circuit 104f is connected between the coil 103b and the smoothing capacitor 103c of the outputting circuit 103. An output terminal of the load current detection circuit 104f, that outputs a detection result of a load current of an electric load 106, is connected to the input terminal A of the switching board 104e. 
Operations of the above-described switching power supply circuit 101 are now described.
When the direct current voltage output by the direct current source 105 is applied to the switching circuit 102 in the normal operation mode while the electric load 106 is connected, a base current generated according to the predetermined frequency is supplied to the normal switching element 102a, the normal switching element 102a performs the switching operations and outputs a pulse to the outputting circuit 103. At this time, the bias current is not supplied to the light load switching element 102b, so that the light load switching element 102b remains inactive.
While the normal switching element 102a is activated, the outputting circuit 103 allows a passage of a current according to a voltage output by the normal switching element 102a, and stores energy in the coil 103b. When the normal switching element 102a turns off, the energy stored in the coil 103b is discharged through the diode 103a. The discharged energy is smoothed by the smoothing capacitor 103c through one cycle of the pulse so that the direct current voltage may be obtained to supply to the electric load 106.
The direct current voltage output by the outputting circuit 103 is input to the differential amplifier 104a of the control circuit 104. The differential amplifier 104a amplifies a difference between the direct current voltage and a voltage obtained by the reference voltage source 104b, and outputs a result of the amplification to the PWM comparator 104c. The triangular wave according to the predetermined frequency generated by the oscillator 104d is input to the PWM comparator 104c. The PWM comparator 104c synchronizes a difference output by the differential amplifier 104a and the triangular wave so as to generate a pulse having a duty according to the difference between the direct current voltage and the voltage obtained by the reference voltage source 104b and outputs the pulse to the switching board 104e. A signal indicating that the load current is in the normal operation is input to the switching board 104e from the load current detection circuit 104f. The switching board 104e selects the output line connected to the normal switching element 102a to supply the base current to the normal switching element 102a with the duty of the pulse output from the PWM comparator 104c. 
The normal switching element 102a reduces the duty when the direct current voltage output by the outputting circuit 103 is higher than a predetermined value and increases the duty when the direct current voltage is lower than the predetermined value, thereby stabilizing the direct current voltage to the predetermined value.
When the electric load 106 changes the status to the light load operation mode such as the standby mode, the load current of the electric load 106 reduces. At this time, the load current detection circuit 104f outputs a signal indicating that the load current is in the light load operation. The switching board 104e selects the output line connected to the light load switching element 102b to supply the base current to the light load switching element 102b with the duty of the pulse output from the PWM comparator 104c. 
Since the light load switching element 102b performs the switching operations in a period shorter than the normal switching element 102a, a power loss due to switching operations may become lower in the light load operation than in the normal operation. When the switching power supply circuit 101 turns to the normal operation again, the normal switching element 102a is reactivated.
As previously described, a NPN-type transistor is used for the normal operation switching element 102a and the light load switching element 102b in the switching power supply circuit 101 of FIG. 1. As an alternative, a MOSFET (metal oxide semiconductor field effect transistor) may be used for the normal operation switching element 102a and the light load switching element 102b, and such technique using the MOSFET as switching elements has also been proposed.
As described above, a switching element having a switching time period shorter than that in the light load operation may be effectively used to reduce the power loss in transition of the switching operation. However, since a power consumed in the switching element is a product of a current in the switching element and a voltage applied to the switching element, the current in the switching element is extremely small in the light load operation, especially in the standby mode. In particular, the current flowing in the switching element is in a range from some μA to some hundred μA, thereby effect on the power loss may be small.
A large amount of power consumption in the light load operation is power consumed to charge and discharge a parasitic capacitance as a drive pulse existing between a control electrode and input and output terminals of the switching element. In a MOS (metal oxide semiconductor) transistor, for example, the parasitic capacitance may exist between a point of a gate and a source of the MOS transistor and a point of the gate and a drain of the MOS transistor.
The power is free from the current in the switching element and remains constant. To reduce the power loss caused by charging and discharging such parasitic capacitance, it is effective to reduce the driving frequency of the switching element.
To achieve the above-described purpose, one technique is used in which a switching frequency is changed to a lower level. In this technique, however, the switching frequency may have a limitation to be reduced no lower than 20 kHz. The frequency below 20 kHz enters an audio frequency band, which gives offensive sound. There is another technique in which the switching operation is intermittently performed. The intermittent switching operation, however, may generate an output voltage having a ripple that needs to be avoided. Furthermore, there is another technique in which a normal switching operation is stopped to drive a series regulator that operates using a current with low power consumption. However, the series regulator includes a voltage control transistor that causes power loss. Even though the series regulator effectively works when a load current in the light load mode is small, the efficiency may become smaller when the load current in the light load mode becomes larger. In addition, a size of the series regulator may cause another problem since adding the series regulator may need a larger scale of a circuit.